thromboxane
Summary
Thromboxane A2 (TXA2) is a prostanoid produced by activated platelets via cyclooxygenase-1 (COX-1) and thromboxane synthase. It promotes platelet aggregation and vasoconstriction, playing a key role in primary hemostasis and arterial thrombosis. Aspirin irreversibly inhibits COX-1, blocking TXA2 synthesis and providing antiplatelet effects.
Detail
Thromboxane A2 is synthesized from arachidonic acid through the cyclooxygenase pathway: phospholipase A2 releases arachidonic acid from membrane phospholipids, COX-1 converts it to prostaglandin H2, and thromboxane synthase (found predominantly in platelets) converts PGH2 to TXA2. TXA2 acts on thromboxane (TP) receptors, which are Gq-coupled, leading to increased intracellular calcium, platelet activation, shape change, and aggregation, as well as vasoconstriction of vascular smooth muscle. This is functionally opposed by prostacyclin (PGI2) produced by endothelial cells, which inhibits platelet aggregation and causes vasodilation via Gs-coupled receptors increasing cAMP. The balance between TXA2 (platelets) and PGI2 (endothelium) is critical for normal hemostasis—disruption favoring TXA2 promotes thrombosis, while excess PGI2 promotes bleeding.
Clinically, aspirin irreversibly acetylates and inhibits COX-1, preventing TXA2 formation for the lifetime of the platelet (since platelets lack nuclei and cannot synthesize new COX-1), thus providing sustained antiplatelet effects—this is why low-dose aspirin is used for cardiovascular prophylaxis. Other NSAIDs reversibly inhibit COX and have a shorter duration of antiplatelet effect. TXA2 is also implicated in the pathophysiology of pre-eclampsia (increased thromboxane relative to prostacyclin causes vasoconstriction and platelet activation) and in bronchoconstriction, though its systemic effects are largely local (autocrine/paracrine) due to its extremely short half-life (~30 seconds) and rapid nonenzymatic degradation to inactive thromboxane B2 (TXB2), which is used as a stable marker for TXA2 production in laboratory studies.
Key associations for boards: Aspirin's mechanism (irreversible COX-1 inhibition → decreased TXA2), Glanzmann thrombasthenia (GpIIb/IIIa deficiency prevents TXA2-mediated platelet aggregation via fibrinogen bridging), and the balance between TXA2 and PGI2 in cardiovascular and obstetric pathology.
Sources
- First Aid for the USMLE Step 1
- Goodman & Gilman's The Pharmacological Basis of Therapeutics
- Robbins Basic Pathology
- Katzung's Basic and Clinical Pharmacology
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